Modern medicine believes that the formation of anal fissure is mainly caused by mechanical stimulation and inflammatory changes in the anal skin and blood vessels. Anatomy: The external anal sphincter is divided into two parts from the tailbone forward to the back of the anal canal. Lack of blood supply and less elasticity in the posterior median line of the anal canal are the causes of anal fissure. 2, injury: dry stool can make the anal skin tear, diarrhea, frequent defecation makes the anal canal sensitive tight, the normal consistency of stool can also cause injury, chronic inflammation of the anal canal, fibrous tissue proliferation, the formation of anal comb hard knots, preventing the sphincter relaxation, so that the anal canal is vulnerable to injury rupture. Rectal foreign bodies, improper anal dilation methods, anal surgery, childbirth, congenital anal stenosis, proctocolitis, tuberculosis, etc. can also lead to anal fissures. 3, infection: acute and chronic anal sinusitis, anal papillitis, internal hemorrhoids and polyps are the main causes of infection. The pathogenic bacteria of infection enter the anal gland through the glandular opening and generate abscesses in the subcutaneous tissue of the anal canal, which form ulcers after breaking down. Small superficial thrombosis occurs due to infection, which can also lead to anal fissure. 4, the etiology of chronic anal fissure: is not clear. It is generally believed that it is related to the following aspects. First, anal fissures are mostly in the lower edge of the posterior sphincter of the anal canal in the shape of a shuttle, with only a small number of edges overlapping with the lower part of the internal sphincter. Secondly, the reflex muscle pain spasm caused by pain makes the blood supply locally insufficient, delaying the healing of anal fissure and becoming chronic anal fissure. Third, there is embryonic residual epithelial tissue deep in the anal canal, and the skin of the anal canal ruptures, exposing the epithelial cells within the submucosa to the bottom of the fissure, and due to the foreign body stimulation of epithelial fragments on the fissure and repeated infection, chronic anal fissure is easily formed.